1. Field of the Invention
The present invention relates to a device for producing signals accurately representing knocking at individual cylinders of an engine, for effecting feedback control of ignition timing thereof in response to the knocking conditions as represented by the signals.
2. Description of the Prior Art
It is known that when excessive knocking is sustained, adverse influences such as a deterioration of engine durability are caused, while a slight knocking state in a comparatively slow range of the engine revolving speed is favorable for optimization of the engine output and fuel cost characteristics. The occurrence of the knocking has a significant correlation with the timing of ignition, and advancement of the ignition timing generally tends to enhance the occurrence of the knocking.
Accordingly, systems which detect the knocking of an engine in order to advance or retard the ignition timing in response to the knocking thus detected, for optimizing the fuel cost and the output characteristics of the engine by allowing slight knocking therein, have been known. A device for detecting the knocking state in the aforesaid systems of the prior art includes a vibratory sensor (an acceleration sensor) adapted to be mounted on an engine body, and the output signal from the sensor is once passed through a band-pass filter for extracting only the knocking frequency band vibratory component, so as to determine the knocking state on the basis of output signals from the band-pass filter.
The conventional knocking detecting device of the aforesaid construction has a shortcoming in that only one vibratory sensor is used and the location of the sensor mount is not necessarily consistent with respect to the individual cylinders of the engine, so that the vibratory level which is recognized by the sensor as knocking varies from cylinder to cylinder, and the accuracy of the device in determining the state of knocking is low.
More particularly, the signals from the vibratory sensor or knock sensor are directly averaged by an averaging circuit having a comparatively large time constant for producing a reference voltage for detecting the knocking, which reference voltage is used for determining the knocking at any of the cylinders of the engine. Thus, at a cylinder where the engine vibration is easily transmitted to the knock sensor, absence of knocking may be falsely judged as presence or occurrence of knocking, and inversely, at a cylinder where the engine vibration is hardly transmittable to the sensor, the presence of the knocking may be falsely judged as the absence of it. Consequently, the detection of the knocking state in the conventional device has been inaccurate.